1. Field of the Invention
This invention relates to a process for the prolonged removal of dust inside a reaction zone on stream. The invention provides further an apparatus being useful in the process.
2. Description of the Related Art
The present invention concerns the dedusting of a feed stream being introduced into a reactor loaded with particles. Dedusting is performed inside the reactor simultaneously with chemical and/or physical reactions.
Those reactions include catalytic and non-catalytic heterogeneous reactions, absorption and adsorption. The dust containing feed may be in the gaseous or liquid form.
Plugging of reactors by dust contained in the feed stream is a well-known problem in the industry. More specifically, in processes involving reactors loaded with one or more beds of particles, dust contained in the feed is deposited on and within particle layers contained in the reactor and finally plugs the reactor. The type of particles loaded usually comprises pellets, cylinders, granules, extrudates, rings or mixtures thereof, of which some may not be active in the desired reaction. Those particles are mentioned as inert particles as opposed to active particles on which the actual reactions perform. Build-up of dust in the reactor primarily takes place in and above the top of the particle layer of the reactor and only to a small degree in the particle layer downstream the inlet end. The depth of dust deposition at top of the particle layer depends amongst other parameters on type of the dust and size of the particles in the layer.
As the dust builds up at top of the particle layer, it gradually fills in the void between the particles and pressure drop across the layer inherently increases. Pressure drop increase is an exponential function of the amount of dust deposited and thus of the time on stream.
Even a small concentration of dust in a feed stream will in time cause the building of a critical pressure drop across the reactor, which makes cleaning or a replacement of the bed layer/s necessary.
Therefore, it may not be a solution to install a dust filters up stream the reactor. This is e.g. the case with hot gases, where electro filters or filter bags are used, thus, where the concentration of dust in the purified stream is typically not reduced to below 2-5 mg/Nm3.
In some processes the time for the building of pressure drop across a reactor to a maximum allowable value is defining the maximum time of continuous operation. In many chemical process plants, it is rather time consuming to close down the plant and restart it, meaning that the cash value of production lost in the meantime is considerable.
It is thus a general object of the invention to prolong the period of continuous operation, thereby increasing the on-stream factor of a plant treating dust containing fluids.
The period of continuous operation limited by the build-up of dust inside a reactor can be prolonged in several ways.
1. The process plant can be designed for a larger pressure drop.
2. Two or more reactors can be placed in parallel, so that one is being cleaned or emptied and reloaded, whilst the other/s are in operation.
3. A larger cross sectional area of the reactor than usually applied can be chosen.
Solution 1 implies an increase of both plant investment and production cost, as the feed stream supplied to the specific plant section in question should be delivered at a higher pressure and/or the down stream section should accept/make up for an effluent stream at a lower pressure than otherwise. Solution 1 will be useful only for a short prolongation, as the pressure drop increases exponentially with operation time.
The drawback of solution 2 is that the investment of the actual reactor is increased considerably. If the reactor in normal operation is at high temperature, further equipment of cooling and reheating is required. Furthermore, in case of a moist dust containing gas feed, condensation may unintentionally occur on cold surfaces during cooling.
In several cases condensation implies operation problems. For instance if the gas contains components which form corrosive substances in the condensate, channels, tubes and equipment subjected to the condensate may be corroded, if these are not constructed from a material resistant to the corrosive conditions. In some processes solids may condense and plug the particle layer, valves or narrow passages in the equipment used for cooling the process gas for regeneration. Condensation may also lead to deterioration of the particles, if these are sensitive to liquid.
The main disadvantage of solution 3 is that a larger cross sectional area leads to inappropriate dimensions of the reactor, especially pressure equipment is disadvantageously imposed a tangent height/diameter ratio, which considerably raises the price of the reactor.
This invention provides a process and a reactor being able to operate on a dust containing streams at a pressure drop profile similar to that of conventional reactor, however, with an increased on-stream factor, by dividing the particle layer of the reactor in at least two divisions (beds) and introducing the feed stream into the reactor above the first and in between each of these divisions.
When distributing the feed gas from the first particle bed to subsequent beds, as the pressure drop builds up from the initial to the maximum allowable in the individual beds, the on-stream period is advantageously prolonged.
Distribution of gas into the reactor is typically performed by means of bypass tubes and valves connecting the reactor sections above each particle bed with the respective reactor section above the following neighbouring particle bed, or alternatively the feed gas tube is connected to a manifold supplying the individual beds of the reactor by adjusting the valves in the individual manifold tubes.
The tube line to the top layer is kept open when hot moist gases are employed in reactors with particles being sensitive to liquid. Opening of the first bypass/admission valve will restrict flow through the first bed, however, not eliminate the flow, which serves to keep all of the particle layers warm and avoid condensation.
By the method and reactor according to the invention, the area in which the dust may build up is increased. When, as preferred, the additional number of installed beds are subjected to the filtering of the dust particles have identical cross sectional area, the maximum operation time on stream is increased proportionately with the additional number of beds. Thus, e.g. by dividing the layer of particles in the reactor into two beds of identical cross sectional area and introducing the main fraction of the feed stream to each of these in series, the area of deposition is doubled.
Preferably, the beds of particles foreseen for dedusting are not much higher than the height of the maximum depth of dust deposition, depending on the type of the actual dust particles.
Advantageously, the valves used for the bypass application need not to be fluid tight. A moderate leakiness of a by-pass valve will have no severe influence on the operation of the reactor, as long as the dust deposited upon/in the particle bed in connection with the leaking by-pass valve does not develop a pressure drop of an order of magnitude comparable to the maximum allowable.
When at the end of an operation period the primary fraction of feed flow is solely directed through the last bed in the reactor, the reactor must still operate satisfactorily. If the reaction taking place is catalytic, the last bed of the reactor must then be able to perform the conversion. In this case all of the bed volume above the last is to be considered extra volume.
When the reaction taking place is absorption, adsorption takes place between components of the fluid and components of the particles, the activity of the particles is reduced with the building up of dust down through the particle bed. Thus, the loss of function of the particles is reduced, when the upper particle beds are bypassed one by one, as at least a fraction of the bypassed particles have already served their purpose. Preferably, the size and shape of particles are chosen such that the loss of the capacity of the particles is minimised.
The sum of heights of the bed layers, corresponding to the volume of the particles, compared to a conventional reactor is larger, meaning that also the height of the reactor is increased. However, compared to solution 3 the reactor is only increased in the direction of its length, which is an increment of reactor volume.